The invention relates, in general, to the fabrication of semiconductor devices, and in particular, to the use of a hydrgoen treatment process to shift the threshold voltage of a Metal Oxide Semiconductor Field Effect Transistor (xe2x80x9cMOSFETxe2x80x9d).
The continual demand for enhanced integrated circuit performance has driven efforts to optimize semiconductor design and processes on every level. Designers seek the ability to optimize performance characteristics of even the most basic circuit components in order to achieve desired results.
Metal oxide semiconductor field effect transistors (MOSFETs) are used extensively in modern semiconductor devices. Various technologies and techniques may be applied during MOSFET fabrication to effect a desired performance characteristic.
A critical performance parameter of a MOSFET device is its threshold voltage. This threshold voltage, which is essentially the voltage required to turn the transistor xe2x80x9conxe2x80x9d, and its response characteristics depend greatly on the MOSFET""s gate material. A variety of different gate materials have been used in the fabrication of MOSFET devices, including doped polysilicon and metals.
An important characteristic of the gate and/or substrate material known as the workfunction. A material""s workfunction, especially that of the gate material, can affect a number of performance parameters in a MOSFET; including the threshold voltage.
In some conventional designs, the gate electrode of a MOSFET comprises a doped polysilicon layer. In conventional MOSFET designs, use of a polysilicon gate typically yields a low difference between the workfunctions of the poly gate and the silicon substrate. The result is a relatively low threshold voltage required to operate the gate. However, certain characteristics of these designs (e.g. poly depletion, resistance) may be undesirable for some designs.
Another conventional approach involves use of metal gates in a MOSFET design. The sheet resistance of a metal gate may be as low as one-tenth that of a poly silicon gate, depending upon the metal and the particular configuration of the device. Additionally, metal gates do not present poly depletion problems. Therefore, commonly used gate metal layers, such as TiN, are advantageous over doped polysilicon gate layers.
A significant disadvantage to the use of TiN and other metal gates used in previous designs has been the difference in workfunction between the metal gate and the silicon substrate. One result is that metal gate MOSFETs have relatively higher threshold voltages, on the order of 0.5 volts, than a comparable polysilicon gate MOSFET.
What is needed, therefore, is a system whereby a metal gate MOSFET can be produced having a lower gate workfunction and, as a result, a lower threshold voltage.
In the present invention, a metal gate MOSFET device is produced with a threshold voltage adapted to a desired value. During fabrication, hydrogen (H) treatment of metal gates immediately after metal deposition is manipulated to selectively shift the threshold voltage (Vt) of a MOSFET to a desired value.
The present invention thus provides a method of producing semiconductor devices, using current semiconductor manufacturing equipment, that more adequately meets the demands of modern high performance integrated circuits.
In one embodiment of the present invention, device processing includes deposition of a metal gate material, such as TiN, and immediately thereafter hydrogen treating the device using a reducing gas. By selectively treating the metal gate material with a H containing reducing gas, the workfunction characteristic of the gate may be lowered and, thus, Vt may be shifted toward a desired value. In one embodiment, the reducing gas is a forming gas. In another embodiment, the reducing gas is silane. In an alternative embodiment, hydrogen treatment is suppressed to effect a desired shift in Vt. 
One embodiment of the present invention provides a process for production of a metal gate metal oxide semiconductor field effect transistor, including the steps of depositing a first metal gate material over gate oxide, treating the device with a reducing gas immediately after deposition of the metal gate, and then depositing a second metal gate material over the treated first. In a preferred embodiment, the first metal gate material is TiN. In one embodiment, the reducing gas is a forming gas. In another embodiment, the reducing gas is silane. In an alternative embodiment, hydrogen treatment is suppressed to effect a desired shift in Vt.
According to the present invention, forming gas treatment is conducted at a temperature of between 400xc2x0 C. and 500xc2x0 C., for a period of from about 10 to about 40 minutes. A preferred embodiment conducts forming gas treatment at about 450xc2x0 C. for a period of approximately 30 minutes. Silane treatment is conducted at a temperature of between 400xc2x0 C. and 500xc2x0 C., for a period of from about 10 to about 40 seconds, at a pressure between approximately 1 and 10 mTorr. A preferred embodiment conducts silane treatment at about 450xc2x0 C. for a period of approximately 30 seconds at a pressure between 1 and 10 mTorr. Depending on the process used and the desired results, reducing gas treatment may be accomplished through a controlled anneal step or, alternatively, an ion implant step.